1. Field of the Invention
The present invention relates to electrochromic devices which exhibit coloration and bleaching thereof at ambient temperature by control of the polarity of an induced electric field. More particularly, this invention relates to a method for providing the area of coloration of such devices by patterning of an insulator on the electrochromic material.
2. Discussion of the Related Art
Electrochromic devices are devices in which a physical/chemical change produced in response to the induced electric field results in a change in the reflective (or transmissive properties) of the device with respect to electromagnetic radiations, e.g., uv, visible and IR radiations. Such devices, one embodiment being shown as item 10 in FIG. 1, generally comprise a film of electrochromic material 12 and an ion-conductive insulating layer 14 which functions as an electrolyte layer. The film and the electrolyte layer are in surface contact with each other for exchange of ions between the electrochromic film and the electrolyte layer. Two conductive electrode layers, 16 and 18 in FIG. 1, at least one of them being transparent, are disposed on the opposite outer surfaces of the film and the electrolyte layer to provide means for applying a voltage across the combined thickness of the electrochromic film and the electrolyte layer. The electrode layers, 16 and 18 in FIG. 1, are provided on substrates, 20 and 22 of FIG. 1, which substrates may be of a material such as glass. Depending on the ion providing and ion storage capacity of ion conductive layer 16, a counter electrode located between ion conductive layer 14 and electrode layer 18 may be used. The electrodes are provided with external electrical leads 24 and 26 connected to a voltage providing source 28. Application of a voltage of proper polarity across the electrodes causes coloration of the electrochromic layer. By reversing the polarity of the applied voltage, the colored electrochromic layer will be uncolored (bleached). Changing from the bleached state to the colored state or from the colored state to the bleached is termed "switching". The electrochromic material may be persistent in either its colored state or its non-colored state. By "persistent" is meant the ability of the material to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "open circuit memory" or simply "memory". In some embodiments, the coloration can be erased by simply short circuiting the electrodes through an external circuit, there being enough internally stored charge to supply the reversed voltage required to raise the coloration in the electrochromic layer. Electrochromic devices of this type have been described for several uses, such as image display, for light filtering, etc. See, e.g., U.S. Pat. Nos. 3,708,220, 4,194,812; 4,278,329; 4,645,308; 4,436,769; 4,500,878; 4,150,879; 4,652,090; 4,505,021; and 4,664,934.
In such devices, the electrochromic film usually comprises an inorganic metal oxide material, most commonly a transition metal oxide, in particular: tungsten oxide. When tungsten oxide is the electrochromic material, the electrolyte layer is adapted to provide a positively charged light metal cation, preferably, a proton or a lithium ion. The electrolyte layer is generally a liquid electrolyte solution which comprises polymers or copolymers containing acidic groups such as polystyrene sulfonic acid or a solid compound like lithium chloride. The electrolyte layer also may be a gel like polyvinyl butyral-methanol doped with LiCl.
When the device is to be used as a display device for portraying specific patterns, either or both of the electrodes is patterned in a manner so as to form the desired pattern. For example, the electrodes may be segmented so as to form an alphanumeric display as is well known in the art and disclosed, e.g., in U.S. Pat. No. 3,955,879 to Witzke et al. One of the problems with patterning in this fashion, is that masking of the substrate to apply the electrodes on only specific areas of the substrate can be difficult and less than commercially desirable. Still further, providing a colored pattern during operation of the device by means of a patterned electrode can provide a colored pattern with diffuse edges due to the diffusion of the electric field at the edges of the patterned electrode. Additionally, controlling patterning by means of patterned electrodes requires that leads be provided to each segment of the patterned electrode which if complex and highly segmented requires complex fabrication techniques. This prior art technique also requires that the leads go from the outer edges of the device and continue through the device between the layers to the patterned electrodes. Not only is it possible that these leads will be undesirably visible through the display device but providing such leads may require complex fabrication techniques which would be less than commercially desirable. As described above, the prior art method does not lend itself to the patterning of areas which are complex, segmented, and detailed in design.
It would be desirable if a method could be found to provide patterning of colorable areas of an electrochromic device, which areas may be complex in shape, segmented and detailed in design, according to techniques less problematic than those used in prior art patterning of the electrode. The invention disclosed herein overcomes the problems of prior art techniques for patterning.